Power batteries (e.g., lithium-ion batteries), due to advantages of high energy density, high power density, long cycling life, long storage time, etc., are widely used in portable electronic devices such as mobile phones, digital video cameras and portable computers, and also have a wide application prospect in electric transportation vehicles such as electric vehicles, electric bicycles and so on, and large-and-medium-sized electric equipment such as energy storage facilities and so on. The power batteries have become a key to solving global problems such as energy crisis and environmental pollution.
When the power battery is overcharged, decomposition of an electrolyte solution in the power battery causes an internal pressure of the power battery to increase, causing fire and explosion to the battery. In order to prevent the fire and explosion caused by an increase of the internal pressure, an external short-circuit unit is generally used and a fuse is placed between a positive electrode and a positive electrode terminal. When the internal pressure of the power battery is increased, the positive electrode and the negative electrode of an electrode assembly are electrically connected into a short circuit by the external short-circuit unit to form a large current which fuses the fuse between the positive electrode and the positive electrode terminal, thereby terminating the charging of the electrode assembly.
However, the above structure reduces tolerance of the power battery against high temperature environment. For example, the above structure greatly increases the probability of thermal runaway of the power battery in a hot-box test. The specific reasons are described as follows. In the high temperature environment, not only the internal pressure of the power battery increases with the decomposition of the electrolyte solution at a high temperature, but also an internal resistance of the electrode assembly increases. In this case, if the positive electrode and the negative electrode of the power battery are electrically connected into a short circuit by the external short-circuit unit, the electrode assembly generates more heat due to the increase of the internal resistance and the short-circuit current, which increases the risk of fire and explosion of the power battery. In other words, the existing external short-circuit unit can only solve the overcharge problem of the power battery under normal temperature conditions, but increases the probability of thermal runaway under a high temperature environment.
In order to reduce the probability of thermal runaway of a secondary battery, the secondary battery in the related art further includes a fuse member made of a low-melting-point alloy, and the fuse member is connected to a negative conduction member to ensure the fuse portion can be melted to cut off the short circuit during the hot-box test. However, when the connection strength between the fuse member and the negative conduction member is low, the contact resistance between the fuse member and the negative conduction member is relatively great, causing the fuse member to fuse when the fuse portion is overcharged to form a short circuit at normal temperature. As a result, even the overcharge problem of the power battery under normal temperature conditions cannot be solved, reducing the safety of the secondary battery.